How important is it to match the number of groomsmen and bridesmaids?

It would be best to match the numbers. It's kind of concerning your fiancee has no one with whom he is close. Is he antisocial? A good suggestion would be to have any of the bridesmaids' boyfriends or husbands fill the role. One of my wife's bridesmaids' husband filled in for one of my groomsmen. Make suggestions on others. If he has no one close to him, he shouldn't mind too much on other men close to you to fill the roles. Best of luck to you.